1. Field of the Invention
The present invention relates to PSK (Phase-Shift Keying) and QAM (Quadrature Amplitude Modulation) modulation techniques for simultaneously transmitting a plurality of bits. The present invention more particularly relates to the detection of the locked condition of PSK or QAM demodulators.
2. Discussion of the Related Art
FIG. 1 schematically represents a conventional QPSK (Quadrature PSK) demodulator. Such a QPSK demodulator is used to extract from a signal Sm two binary signals I and Q modulated in phase quadrature. Signal Sm is generally expressed by I cos .omega.t+Q sin .omega.t, where cos .omega.t and sin .omega.t are two carriers having the same frequency .omega./2.pi., but are in phase quadrature. Signal Sm is applied to two multipliers 10 and 11 which further receive signal cos .omega.t and signal sin .omega., respectively, provided by a voltage controlled oscillator (VCO) 13 connected in a phase-locked loop (PLL). If the frequency of oscillator 13 is close to the frequency of the carrier, multiplier 10 provides a signal having a mean value corresponding to signal I and an a.c. component whose frequency is twice the carrier's frequency. Similarly, multiplier 11 provides a signal having a mean value corresponding to signal Q and an a.c. component whose frequency is twice the carrier's frequency. Low-pass filters 15 eliminate the a.c. components of the outputs of multipliers 10 and 11 and respectively provide signals I and Q.
A phase detector 17 receives signals I and Q and provides a signal .phi. indicative of the phase error of signals I and Q. Signal .phi. controls the frequency of oscillator 13 so that the phase difference .phi. tends to zero. Generally, signal .phi. is applied to oscillator 13 through a low-pass filter 19 whose cut-off frequency is very low so that oscillator 13 is only controlled by the mean variations of signal .phi..
FIG. 2A illustrates a conventional representation, in the form of a "constellation", of the possible combinations of the demodulated binary signals I and Q. The values of signal I are read along a horizontal axis I, and the values of signal Q are read along a vertical axis Q. In QPSK modulation, each signal I and Q has a positive value or a negative value of the same amplitude, corresponding to the high and low logic levels. The nominal constellation, corresponding to the case where the signals provided by oscillator 13 are in phase with the two carriers in quadrature, includes four points P1-P4 that are symmetrical with respect to axes I and Q.
When the signals of oscillator 13 are phase shifted by an angle .phi. with respect to the carriers in quadrature, one obtains an effective constellation rotated by an angle .phi. with respect to the nominal constellation P1-P4, as shown in FIG. 2A. In addition, if the frequency of oscillator 13 differs from the frequency of the carrier, angle .phi. increases, i.e., the effective constellation rotates, at a speed equal to the frequency difference between the carrier and the oscillator 13.
FIG. 2B represents the phase error variation .phi. when this phase difference is equal to .DELTA.F. Signal .phi. is a saw-tooth varying between -.pi./4 and +.pi./4 at a frequency equal to 4.DELTA.F (the effective constellation reaches its nominal condition at each quarter of turn).
If the frequency difference .DELTA.F is low (within the passband of filter 19), the control signal of oscillator 13 follows the variation of signal .phi. and rapidly adjusts the frequency of oscillator 13 so that signal .phi. does not reach one of the limits -.pi./4 or +.pi./4. In contrast, if the frequency difference .DELTA.F is important (much higher than the cut-off frequency of filter 19), signal .phi. varies so rapidly that the control signal of oscillator 13 cannot vary at the same speed. Then, signal .phi. becomes a saw-tooth as represented in FIG. 2B, and the control signal of oscillator 13 establishes at the mean value, i.e. zero, of this saw-tooth signal. As a consequence, oscillator 13 is in a steady state but is set at an erroneous frequency.
To avoid this situation, lock detection circuits are used for directly analyzing signal .phi. and for indicating a lock condition when the amplitude of signal .phi. is between two thresholds. When signal .phi. exceeds these thresholds, oscillator 13 is forced, for example, to scan through its frequency range until a signal .phi. varying below these thresholds is obtained.
However, signal .phi. is often excessively noisy, which does not allow it to be compared with a threshold.